See-through displays such as head-up instrument displays, head- or helmet-mounted displays, and related devices allow the user to simultaneously view the physical world and an electronically-generated and optically projected image of text, graphics, or other video material. Brightness variations in the physical scene often make it difficult to achieve proper brightness in the projected image so that both the physical scene and the projected image are properly visible throughout the scene.
Consider for instance the situation where a pilot""s field of view encompasses sunlit regions outside of the cockpit as well as more dimly illuminated regions within the cockpit. If the projected image overlays all of these regions, a single global adjustment of the projected image brightness will be inadequate. If such a global adjustment were set so that the projected image is visible against the bright portions of the physical scene, the dimmer portions of the physical scene will be very difficult to see, if not masked altogether. If, on the other hand, the projected image brightness is adjusted to provide adequate brightness against the dimly illuminated regions, the projected image will not be visible against the brightly-illuminated portions of the physical scene. An optimal solution requires that the brightness of each display pixel or zone be adjusted on the basis of the brightness of the corresponding portion of the physical scene or background. Since the brightness of the pixels or zones of the physical scene can vary dynamically across the entire scene, the brightness of the corresponding portions of the projected image must be adjusted in real time to account for such changes.